Cooling multi-chip modules using embedded heat pipes

ABSTRACT

A method and apparatus is disclosed for cooling a multi-chip module using embedded heat pipes. Semiconductor chips are disposed into the multi-chip module through cavities in the module substrate. The semiconductor chips engage heat pipes embedded within the substrate. The heat pipes conduct heat away from the semiconductor chips through a heat conductive bonding layer.

This is a division of application Ser. No. 07/961,153, filed Oct. 15,1992 now U.S. Pat. No. 5,268,812, which is a continuation of applicationSer. No 07/749,575, filed Aug. 26, 1991 now abandoned.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to cooling of a multi-chip module. Moreprecisely, the present invention relates to embedding heat pipes intothe substrate of a multi-chip module to cool semiconductor chips mountedon the module.

2. Art Background

An important objective of computer design is to fit the greatest numberof semiconductor chips or ICs into the smallest space. Factors such assubstrate design, interconnect design, cooling method, density of chipplacement, etc., have great bearing on the ultimate performance of thecomputer. The tendency of designers to minimize the size of the computerwhile maximizing its computing power has led to more and more denselypacked IC chips. The density of interconnects that provide the signalpath between ICs must concurrently rise. Unfortunately, these denselynetworked interconnects have a propensity to generate heat.

By the same token, higher computing power translates to a faster rate atwhich instructions are executed. To execute more instructions persecond, the circuits must operate at a higher frequency. Operating at ahigher frequency requires higher energy input and consequently moreenergy is generated in the devices. A byproduct of high energy input isheat.

Higher computing power also means the ability to execute larger andlarger sets of instructions. As a result, the semiconductor devices usedwithin a given area must have greater memory capacity to accommodate theincrease. Thus, more energy is required to operate the increased numberof memory devices. Again, more energy input results in more energyappearing as heat. It follows then that cooling of these devices shouldbe a major concern.

In prior art computers, the circuits were simply cooled by airconvection circulated by a fan. But when the fan was used in conjunctionwith high density, multi-chip, main frame computers, the large volume ofair needed for cooling necessitated powerful blowers and large ducts.Such clumsy structures in the computer occupied precious space and werenoisy too.

There have been other approaches to cooling ICs. For example, U.S. Pat.No. 4,748,495 to Kucharek discloses a package for cooling a high densityarray of IC chips and their interconnections. In this arrangement, theIC chips are mounted in a generally planar array with individual heatsinks connected to the ICs separated by flexible membrane mounts. All ofthe cooling structure are thus mounted on top of the ICs, separated bythe membrane mounts. Cooling fluid is then pumped through the coolingstructures, thus carrying coolant past the areas above the ICs.

Likewise, U.S. Pat. No. 4,879,629 to Tustaniwskyj et al. discloses aliquid cooled multi-chip integrated circuit module that incorporates aseamless compliant member for leak proof operation. In particular, heatsinks are disposed immediately on top of the ICs while on top of theheat sinks are disposed channels that carry liquid coolant wherein thechannel is incorporated into a rigid cover. A compliant member seals offthe channel area from the chip area to eliminate the possibility ofleakage of liquid coolant.

Because of variations in the way ICs are mounted to the substrate, thetop surface of the IC may be tilted at different angles which impairsheat conduction to the heat sink. U.S. Pat. No. 5,005,638 to Goth et al.provides structure to ensure solid contact between the heat sink and theIC. Goth discloses barrel shaped pistons that are spring loaded andbiased toward the IC chips such that any minor tilt in the chips arecompensated by the springs. Heat then rises from the IC chips up throughthe barrel shaped pistons and into a large body heat sink. Coolant isthen pumped through the heat sink to assist in heat dissipation.

Unfortunately, with the coolant fluid disposed above the chips as in theprior art, there is always a possibility of coolant leakage. If suchleakage should take place, assuming the coolant is electricallyconductive, the malfunction would be catastrophic. Even if the coolantwere not conductive, it would contaminate the chips leading to otherreliability problems. Furthermore, the structures needed for conductionof fluid and contact between the heat sink and the chips are typicallyvery complex. These intricate structures require a great deal ofattention during assembly and are usually expensive to fabricate.

Finally, U.S. Pat. No. 5,095,404 to Chao discloses a cooling apparatusmounted to a printed circuit board. The printed circuit board has a holefor receiving the cooling apparatus comprising several layers includinga heat spreader, a heat pipe, and heat sinks. The heat spreader engagesa semiconductor chip through the hole in the printed circuit board. Theheat spreader is mounted to the under side of the printed circuit board,and the heat pipe is mounted to the heat spreader through interveninglayers. Heat sinks are coupled to the heat pipe. Unfortunately, such amulti-layer cooling apparatus does not conduct heat efficiently sinceeach layer adds a resistive barrier to heat conduction. Moreover, suchan externally mounted cooling apparatus increases the space required bythe printed circuit board, thereby increasing the overall form factor ofthe system containing the printed circuit board.

SUMMARY OF THE INVENTION

The present invention is a method and apparatus for cooling a multi-chipmodule using embedded heat pipes. The embedded heat pipes conduct heataway from semiconductor chips that are disposed into the multi-chipmodule through cavities in the module substrate.

For one embodiment, a module cooling apparatus comprises a substratehaving an integral thermal conduction layer coupled to a heat pipe. Thesubstrate is formed with a plurality of cavities for receivingsemiconductor chips through a top face of the substrate. The thermalconduction layer is embedded into a bottom face of the substrate. Thethermal conduction layer has a top surface for engaging with thesemiconductor chips through the cavities. A heat pipe for absorbing heatfrom the thermal conduction layer engages a bottom surface of thethermal conduction layer. Heat generated by the semiconductor chips isconducted through the thermal conduction layer and dissipated in theheat pipe.

For another embodiment, the module cooling apparatus comprises asubstrate with a plurality of heat pipes embedded within the substrate.A plurality of cavities for receiving semiconductor chips are formedthrough a top face of the substrate. Each of the heat pipes containedwithin the substrate has a top surface for engaging with thesemiconductor chips through a corresponding cavity. The heat generatedby the semiconductor chips is conducted through a thermoplastic bondinglayer and is dissipated in the heat pipes.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view of a multi-chip module having a heatsink integrated into the substrate.

FIG. 2 is a cross-sectional view of a multi-chip module having anintegrated heat pipe joined to a heat sink.

FIG. 3 is a perspective view of an example multi-chip module with a setof semiconductor chips disposed into cavities in a top face of themodule substrate.

FIG. 4 is a top view of the example module, and shows the semiconductorchips disposed within a set of cavities in the top face of thesubstrate.

FIG. 5 is a side view of the example module showing the semiconductorchips disposed into the substrate, and showing the heat pipes embeddedwithin the substrate and running along the length of the module.

FIG. 6 is a cross sectional view showing a portion of the examplemodule, and showing a semiconductor chip disposed within a cavity in themodule substrate.

DETAILED DESCRIPTION OF THE INVENTION

The following specification describes a method and apparatus for coolinga multi-chip module using embedded heat pipes. In this description,specific materials and configurations are set forth in order to providea more complete understanding of the present invention. But it isunderstood by those skilled in the art that the present invention can bepracticed without those specific details. In some instances, well-knownelements are not described precisely so as not to obscure the invention.

The present invention integrates a heat pipe directly into a multi-chipmodule (i.e., MCM) substrate, and is thus not simply bolted on. This isdistinct from the prior art, which add discrete cooling structures tothe MCM substrate. By embedding the heat pipe directly to the MCM, it ismuch simpler for product assembly and possible rework.

An MCM package cooled by the present invention is not as prone tocoolant leakage because the coolant is contained within the substratethus minimizing the possibility of contamination with the chips.Further, the simplified construction of the cooling mechanism ensuresthat the present invention is less expensive to build than the prior artcooling devices. Also, positive contact between the cooling mechanism ofthe present invention and the IC chips results in efficient heatconduction as compared to the prior art devices that required variousmake-shift hardware to obtain positive contact.

Referring now to FIG. 1, a multi-chip module just prior to installationof the heat pipe is illustrated. A packaging substrate 12 is provided tohold a plurality of semiconductor chips 10. For one embodiment, thepackaging substrate 12 contains a network of cavities 34 which extendcompletely through the thickness of the substrate 12. Through variousmethods known in the art, semiconductor chips 10 are inserted into thesecavities 34. Electrical interconnects (not shown) provided on the chips10 and the packaging substrate top face 26 allow electricalcommunication among the chips 10 and facilitate electrical interfacewith external devices. For this purpose, conventional wire interconnectscan be soldered to the chips 10. Or more exotic Tape Automated Bonding(TAB) techniques can be used to form the interconnects, as disclosed inU.S. Pat. No. 5,121,293 to Conte. After installation, the semiconductorchips 10 should preferably be recessed into the substrate 12 such thattheir bottom surfaces 36 are exposed. A thermal conduction means 14,embedded into the bottom face 28 of the packaging substrate 12, isadapted to engage the bottom surfaces 36 of the chips 10. Aninterference fit is sufficient to mechanically hold the thermalconduction means 14 in place. Other means of attachment such as cement,mechanical fasteners, or other means known in the art are suitable tohold the conduction means 14 in position. Positive engagement is thusobtained between the bottom 36 of each chip 10 against the thermalconduction means 14. For one embodiment, the thermal conduction means 14is a copper slug. Clearly, other thermoconducting devices known in theart are acceptable in place of the copper slug.

A bonding process is used to insure proper heat flow from thesemiconductor chip 10 to the thermal conduction means 14. This dieattach process involves supplying a quantity of heat conductivethermoplastic material to join the die 10 to the thermal conductionmeans 14. At first, the thermoplastic material is fluid and flows tofill any voids in the joint. After curing, the bond stiffens insofar asno further flow occurs, but the joint remains flexible. For oneembodiment, the thermoplastic material should be hexagonal boronnitride. Significantly, the compliant nature of the thermoplasticmaterial maintains a solid thermal bond even when the packagingsubstrate 12 undergoes flexing during assembly.

For one embodiment, a heat pipe 38 dissipates heat accumulating in thethermal conduction means 14. The heat pipe 38 is preferably made fromcopper or aluminum. For efficient heat transfer via conduction, it ispreferred that the heat pipe 38 be directly and positively attached tothe thermal conduction means 14. Conventional mechanical fasteners suchas clamps of screws can be used to lock the two structures together.Further, there should be no air gaps between the contacting surfaceswhich would impair heat conduction.

The heat pipe 38 functions similarly to a miniature refrigerator (notshown). In a typical closed circuit refrigeration cycle, the heattransfers to an evaporator which vaporizes a liquid refrigerant causingit to travel into a compressor, which in turn moves the high pressurevapor into a condenser. The condenser transfers heat out of the systemand condenses the refrigerant back to liquid form. Thereafter, thecondensate travels through a pressure-lowering expansion valve back intothe evaporator, where the cycle repeats. The principles used here arewell-known in the art and need not be explained in further detail.

Those very same principles apply in the heat pipe 38 utilized in thepresent invention. Inside the hollow casing 24 of the heat pipe 38 is achamber containing refrigerant or coolant 18 and a wick 16. For oneembodiment, the coolant 18 is a dielectric fluorocarbon. As shown inFIG. 2, one side of the heat pipe 38 is placed against the bottomsurface 32 of the thermal conduction means 14 while the opposite side ofthe heat pipe 38 faces away. The proximal region of the heat pipe 38closest to the thermal conduction means 14 operates as an evaporator 22,while the distal region furthest away functions as a condenser 20.Accordingly, as heat travels from the thermal conduction means 14 intothe evaporator 22, the coolant 18 in that region is vaporized and movesaway from the evaporator 22 to the condenser 20. In the condenser 20,the vapor cools and transitions back to its liquid form after heat isdissipated through casing 24 and out through a radiating surface 40.

After condensing to liquid form, the coolant 18 is drawn through a wick16 back to the evaporator 22 by capillary action. As is known in theart, capillary action is due to surface tension; namely, cohesion of theliquid molecules and the adhesion of the molecules on the surface of asolid. When the adhesion is greater than the cohesion, the liquid isdrawn along the wick from the wet side to the dry side. Consequently, aclosed loop cycle is established in that the coolant 18 is evaporated inthe evaporator 22, moves to the condenser 20 where it condenses to aliquid, and is finally drawn into the wick 16 to be carried back to theevaporator 22.

For one embodiment, the packaging substrate 12 along with the appendeddevices are inverted during final assembly to a motherboard (not shown).Therefore, surface 26 along with the semiconductor chips 10 all facedownward toward the motherboard. As a result, the heat pipe 38 ispositioned at the highest point on the substrate package 12. Thecondenser 20 of the heat pipe 38 is thus ideally located at a pointhigher in elevation than the evaporator 22. This particular orientationis conducive to optimal heat transfer and radiation away from the heatgenerating device 10 since heat travels upward; and as the light-weightevaporated coolant 18 rises from the evaporator 22, the denser andheavier condensed liquid form of the coolant 18 is drawn back to theevaporator 22 by gravity, as well as through operation of the wick 16.An optional second heat sink 42 with fins 44, which structure is wellknown in the art, can be added to the radiating surface 40. Theforegoing is only one arrangement for mounting the heat pipe 38 to thethermal conduction means 14.

In an alternate arrangement (not shown), the heat pipe can extend to oneside of the thermal conduction means. In this disposition, the entireregion proximal to the thermal conduction means functions as aevaporator while the distal portion away from the thermal conductionmeans and not in engagement therewith functions as a condenser.

FIG. 3 is a perspective view of an example multi-chip module 100. Thesemiconductor chips 120-123 are disposed into a top face 114 of asubstrate 110 through cavities. A group of heat pipes 150-171 arecontained within the substrate 110. For the example module 100, the heatpipes 150-171 run along the length of the module 100.

FIG. 4 is a top view of the example module 100. The semiconductor chips120-123 are disposed within a set of cavities 140-143 formed in the topface 114 of the substrate 110. The cavities 140-143 expose uppersurfaces of the heat pipes 150-171. The semiconductor chips 120-123 aredisposed into the substrate 110 and engage one or more of the heat pipes150-171. The heat pipes 130 dissipate heat generated by thesemiconductor chips 120-123. For one embodiment, it is preferable thatthe heat pipes 130 be made from copper or aluminum.

For one embodiment, the heat pipes 150-171 have closed ends that extendthrough a surface 112 of the substrate 110. However, for otherembodiments, the heat pipes 150-171 are completely confined within thesubstrate 110 according to the thermal arrangement of the systemcontaining the module 100.

The heat pipes 150-171 function in a similar manner to the heat pipe 38discussed above. The heat pipes 150-171 each have a chamber containingrefrigerant or coolant and a wick. For one embodiment, the coolant is adielectric fluorocarbon. The regions of the heat pipes 150-171 closestto the semiconductor chips 120-123 function as evaporators, while theregions of the heat pipes 150-171 farthest away from the semiconductorchips 120-123 function as condensers.

Heat travels from the semiconductor chips 120-123 into the evaporatorsof the heat pipes 150-171. Thereafter, the coolant near thesemiconductor chips 120-123 is vaporized and moves away from theevaporators toward the condensers of the heat pipes 150-171. In thecondensers, the coolant vapor cools and transitions back to a liquidform as heat is dissipated through the substrate 110 and through theends of the heat pipes 150-171 extending through the surface 112. Aftercondensing to liquid form, the coolant is drawn through the wicks to theevaporators by capillary action, thereby establishing a closed loopcycle.

FIG. 5 is a side view of the example module 100. The semiconductor chips120-121 are shown disposed into the top face 114 of the substrate 110.Also shown are the heat pipes 150-171 running along the length of themodule 100. The semiconductor chip 120 is disposed within the cavity 140formed in the top face 114 of the substrate 110. Similarly, thesemiconductor chip 121 is disposed within the cavity 141 formed in thetop face 114 of the substrate 110. The semiconductor chips 120-121engage the upper surfaces of the heat pipes 151-156.

FIG. 6 is a cross sectional view showing a portion of the example module100. The semiconductor chip 121 is shown disposed within the cavity 141.The cavity 141 is formed through the top face 114 of the substrate 110.Also shown are the heat pipes 150-156 as viewed through the surface 112of the substrate 110.

A contact junction 144 is formed between the semiconductor chip 121 andthe substrate 110, and between the semiconductor chip 121 and the uppersurfaces of the heat pipes 151-156. The heat generated by thesemiconductor chip 121 travels through the contact junction 144 and intothe evaporators of the heat pipes 151-156. The heat generated by thesemiconductor chip 121 is dissipated in the heat pipes 151-156 in themanner described above.

For one embodiment, a heat conductive thermoplastic material is appliedto the contact junction 144 to bond the semiconductor chip 121 to thecavity 141. The heat conductive thermoplastic material also ensuresefficient heat flow from the semiconductor chip 121 through the contactjunction 144 to the upper surfaces of the heat pipes 151-156. At first,the thermoplastic material is fluid and flows to fill any voids in thecontact junction 144. After curing, the bond stiffens insofar as nofurther flow occurs, but the joint remains flexible. For one embodiment,the thermoplastic material is hexagonal boron nitride. The compliantnature of the thermoplastic material maintains a solid thermal bond evenwhen the substrate 110 undergoes flexing.

Heat pipes of other configurations known in the art are suitable and,depending upon space constraints or particular heat transferapplication, their orientation can be easily varied according to thepresent invention. Indeed, many variations thereon are possible so theactual scope of the disclosure should be determined by reference to theappended claims.

What is claimed is:
 1. A method for cooling semiconductor chips,comprising the steps of:encasing a plurality of heat pipes within asubstrate for mounting the semiconductor chips, each heat pipe having afirst end and a second end; sealing the first and second ends of eachheat pipe and enclosing the first and second ends within the substrate;forming a cavity for each semiconductor chip through a top face of thesubstrate, the cavities exposing a top surface of the heat pipes;coupling the semiconductor chips through the cavities in the substratesuch that each semiconductor chip engages the top surface of the heatpipes.
 2. The method of claim 1, wherein a contact junction is formedbetween the heat pipes and each semiconductor hip at the top surface ofthe heat pipes, such that heat generated by the semiconductor chipstravels through the contact junctions and is dissipated in the heatpipes.
 3. The method of claim 2, wherein the step of coupling thesemiconductor chips through the cavities in the substrate includes thestep of bonding the semiconductor chips to the top surfaces of the heatpipes at the contact junctions.
 4. The method of claim 3, wherein thestep of bonding the semiconductor chips to the top surfaces of the heatpipes at the contact junctions includes the step of applying a compliantheat-conducting thermoplastic material to the top surfaces of the heatpipes at the contact junctions.
 5. The method of claim 2, wherein eachheat pipe comprises a hollow casing containing a coolant, an evaporatorportion, a condenser portion, and a wick, such that the evaporatorportion defines a proximal region of the hollow casing substantiallyadjacent to the cavities, the condenser portion defines a distal regionof the hollow casing away from the evaporator portion, the wick extendsa length inside the hollow casing between the evaporator portion and thecondenser portion.
 6. The method of claim 5, further comprising thesteps of:evaporating the coolant in the evaporator portion using theheat generated by the semiconductor chips; transferring the coolant tothe condenser portion; drawing the coolant in the condenser portion backinto the evaporator portion through the wick by capillary action.